Getting the current thread ID without a syscall?

[Trent Nelson]

On Tue, Jan 15, 2013 at 02:33:41PM -0800, Ian Lepore wrote:
> On Tue, 2013-01-15 at 14:29 -0800, Alfred Perlstein wrote:
> > On 1/15/13 1:43 PM, Konstantin Belousov wrote:
> > > On Tue, Jan 15, 2013 at 04:35:14PM -0500, Trent Nelson wrote:
> > >>
> > >>      Luckily it's for an open source project (Python), so recompilation
> > >>      isn't a big deal.  (I also check the intrinsic result versus the
> > >>      syscall result during startup to verify the same ID is returned,
> > >>      falling back to the syscall by default.)
> > > For you, may be. For your users, it definitely will be a problem.
> > > And worse, the problem will be blamed on the operating system and not
> > > to the broken application.
> > >
> > Anything we can do to avoid this would be best.
> > 
> > The reason is that we are still dealing with an "optimization" that perl 
> > did, it reached inside of the opaque struct FILE to "do nasty things".  
> > Now it is very difficult for us to fix "struct FILE".
> > 
> > We are still paying for this years later.
> > 
> > Any way we can make this a supported interface?
> > 
> > -Alfred
> 
> Re-reading the original question, I've got to ask why pthread_self()
> isn't the right answer?  The requirement wasn't "I need to know what the
> OS calls me" it was "I need a unique ID per thread within a process."

    The identity check is performed hundreds of times per second.  The
    overhead of (Py_MainThreadId == __readgsdword(0x48) ? A() : B()) is
    negligible -- I can't say the same for a system/function call.

    (I'm experimenting with an idea I had to parallelize Python such
     that it can exploit all cores without impeding the performance
     of normal single-threaded execution (like previous-GIL-removal
     attempts and STM).  It's very promising so far -- presuming we
     can get the current thread ID in a couple of instructions.  If
     not, single-threaded performance suffers too much.)

        Trent.